If you book A and C, hoping B will remain empty, and it does not...whose choice is it where the B person goes (aisle or window) - his or yours?

Not really related to AA, but it is COMPLETELY the choice of the pax assigned to B whether or not s/he wants to change seats. Yes, most people will change seats - but don't simply assume B pax will.

You simply ask which they would prefer and let them choose. My guess is the aisle would be the most likely selection.

Trading seats sometimes becomes a question of "Let's make a deal" and what the "B" person wants out of the deal.

If "B" refuses to trade seats and a couple is separated, it can make for a miserable flight.

People who are experienced at flying with often not trade one "B" seat for another unless asked to do so by a gate agent.

If you select A and C, then you should be prepared to sit in A and C. If you are hoping that the person who is in B is kind enough to switch seats with one of you, I would think that you'd make the offer as attractive as possible by offering them either seat.

I think most people would be happy to get out of the middle seat, but some people might find it irritating that you assume that they will give up their seat for you while acting as if you're doing them this big favor.

I use this strategy often.
I have yet to meet a person who wouldnt be happy to switch out of the middle.

Yeah, I've seen many instances of passengers asking another to exchange their aisle seat for a middle seat so the other party can sit together. In most cases that I've seen, they'd get upset at the thought of being asked or deny that request (probably frequent flyers). But every once in a while someone will try to help out at their expense of own comfort. Don't count on asking for aisle to window moves either!

To the OP, since you're requesting the change, it'd be up to the other passenger to move from his/her assigned seat. I did, however, one time notice a FA ask this middle passenger in E+ (or exit row, I can't remember) to see if she'd like to move to a row in the back to an aisle seat with an empty middle seat. At first FA's "recommendation", she declined. Upon a second FA's comment/"request", she decided to move to the back leaving an empty middle seat.

We DID tell the Newcomer B that we were together and would he prefer the aisle or window and it was just what he said that surprised us...he said, quite alarmedly, "I don't want THAT window!"...now, I didn't mention we were in the exit row, we sort of got the feeling he was looking for the comfort without the responsibility, that's all. We would have gone either way, and so we did move over to A and B. I mentioned that I do have to get up a couple of times to use the washroom and he said he was fine with that, then proceeded to fall asleep so I had to wake him up a couple of times.

This is not at all related to a specific airline, so it will exchange seats and move from American AAdvantage to a nice, new seat in TravelBuzz! - or be merged into an undoubtedly existing thread there. Please continue discussion there.

Depending on the plane, I may have also declined an exit row window seat. Some of them can get VERY cold throughout the flight...

If you want to sit together, then book AB or BC. I will happily keep B if I think it will inconvenience people who have been trying to game me.

In a 3-3 configuration we always book C D. No use if you want to hold hands or snuggle up, but if you simply want to talk to your travel companion then having an aisle between you is no obstacle.

While I agree that it's the middle person's choice, that doesn't mean the AC people can't influence that choice. They don't have to open the discussion with "would you like A or C?" If they prefer, say, AB, the C person can simply ask the B person if he or she would like to switch for the aisle without mentioning (yet) that the window could also be an option. Most will switch, since most people consider that an improvement. If the answer is "no," they can try offering the window to see if that offer gets better results.

"Thanks, but I like to sit in the middle seat so I can spread my elbows out over both arm rests."

Everyone got the seat on their boarding pass so no one is obligated to move. I have seen a person sitting in the middle seat decline to move. You can't expect that people will just move to accommodate you, even if you think you are giving them a better seat.

Booking seats hoping the middle won't be occupied isn't working as much these days. The flights are fuller than ever. Himself and I usually book across the aisle from each other. That way we can share stuff and still talk, but neither is stuck in a middle seat.